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Extended Forecast Discussion
 
(Latest Discussion - Issued 1908Z Apr 15, 2024)
 
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
257 PM EDT Mon Apr 15 2024

Valid 12Z Thu Apr 18 2024 - 12Z Mon Apr 22 2024


...Overview...

Guidance generally shows a transition toward more progressive flow
after the start of the period this Thursday when an Omega block
pattern prevails over the northeastern Pacific through
northwestern North America into the northern tier U.S. Ahead of a
southern Canada upper low, the front trailing from surface low
pressure exiting the Great Lakes will continue to bring some
showers and storms across parts of the East. Then the combination
of opening/progression of the upper low and trailing shortwave
energy should bring some troughing into the East by the weekend and
early next week. This evolution may cause a secondary cold front
to decelerate or briefly stall across the Central Plains-MS Valley
region, and aided by weak shortwave energy through the
Southwest/Southern Plains, may help to enhance precipitation from
the Middle-Lower Mississippi Valley into southern Plains late this
week into the weekend. Meanwhile northeastern Pacific energy
undercutting the ridge over western Canada should bring a front
into the Northwest during the weekend and possibly the northern
Plains thereafter. However a fair amount of spread develops for
eastern Pacific into northern tier flow by late weekend/early next
week.


...Guidance/Predictability Assessment...

Latest guidance agrees fairly well for the overall evolution
aloft across southern/eastern Canada into the lower 48, as the
initial upper low over southern Canada opens up and progresses
eastward while the combination of trailing cyclonic flow and a
southern stream shortwave reach the central-eastern U.S. by Sunday-
Monday. Shortwave differences are smaller in scale and thus have
low predictability multiple days out in time. There are more
notable differences in southward progression of the southern tier
surface front and location of heaviest QPF. 00Z ECMWF-initialized
machine learning models (MLs) offer support for at least greater
frontal/QPF suppression than the 00Z ECMWF if not even toward the
GFS, while latest operational dynamical model runs are clustering
toward ECMWF ideas for a time over the southern Plains and
vicinity. The new 12Z ECMWF has nudged the front southward over the
Southeast especially around Saturday. By next Monday there is
reasonable agreement and continuity regarding a frontal wave
developing off the East Coast in response to the upper trough
reaching the eastern U.S.

Latest dynamical models have maintained recent trends toward
Pacific trough energy (with possible embedded upper low) reaching
southwestern Canada and the northwestern U.S. by late Saturday or
early Sunday with continued progression near the Canadian
border thereafter. The 00Z MLs on average suggest slower
progression of this energy versus the dynamical runs, but the new
12Z GFS/CMC runs maintain similar timing from the prior cycle. The
new 12Z ECMWF has trended a little slower though. Ensemble
spaghetti plots become chaotic not only with this system but also
upstream over the eastern Pacific, lowering confidence in details
to the immediate west of the aforementioned feature as well.

The guidance distribution where discrepancies exist generally
favored an intermediate blended approach. Thus the updated forecast
based on 00Z/06Z runs incorporated an operational model composite
early-mid period, followed by modest incorporation of the 06Z
GEFS/00Z ECens means by days 6-7 Sunday-Monday.

...Weather/Hazards Highlights...

Some generally light precipitation is likely on Thursday over parts
of the Northeast/Mid-Atlantic with a leading cold front. The next
front right behind it will renew rain and convection across the
East late this week, while lingering across the south-central U.S.
as the frontal system slows down and possibly stalls briefly.
Anomalous moisture and instability present in the vicinity of the
wavy front favors a low-end Marginal Risk area from near the
confluence of the Ohio/Mississippi Rivers southwest through parts
of Texas on one or both days (Thursday/Friday) covered by the Days
4-5 Excessive Rainfall Outlooks. This overall area should have some
potential for storm training with higher rain rate potential,
though guidance signals do not show great coherence for details
yet. Also most of this area has been rather dry as of late, which
should inhibit the overall flash flood threat. Weak shortwave
energy reaching into the Southern Plains and low level upslope flow
north of the surface front should help to further enhance
precipitation across Texas and vicinity on Saturday with increasing
moderate to heavy rain potential. Then some rainfall should move
into the Southeast as the Northeast dries out. The system currently
expected to brush the Northwest during the weekend may bring some
light to locally moderate precipitation.

Expect a broad area of above normal temperatures by 5-15 degrees
over the southern half of the country into Thursday, while the
northeastern quadrant of the U.S. gradually cools behind a cold
front. Meanwhile the upper trough across the northern tier states
will support below normal temperatures (with greater anomalies for
highs than lows) over the northern-central Rockies/Plains as of
Thursday. Cooler temperatures will gradually expand farther south
and east behind the secondary cold front late week into the
weekend, with near to below average temperatures expected to reach
everywhere east of the Rockies but Florida by next weekend. Coolest
anomalies will be for highs up to 15-20 degrees below normal over
parts of the High Plains. Meanwhile, above normal temperatures
across the Southwest should gradually expand northward underneath a
building Western U.S. ridge and progress eastward into the Rockies
early next week. Northern parts of the West may stay near normal
due to the upper shortwave/cold front crossing the region this
weekend.


Rausch/Santorelli


Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation forecast (QPF), excessive rainfall
outlook (ERO), winter weather outlook (WWO) probabilities, heat
indices, and Key Messages can be accessed from:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml
https://www.wpc.ncep.noaa.gov/#page=ovw